1. Field of the Invention
This invention relates to geldanamycin compounds and methods for their preparation and use, in particular where extracellular heat shock protein 90 is inhibited.
2. Description of Related Art
Geldanamycin belongs to the ansamycin family of natural products, whose members are characterized by a benzenoid nucleus (typically a benzoquinone or hydroquinone nucleus) connected at two meta positions to form a macrocyclic lactam. Besides geldanamycin, the ansamycins include the macbecins, the herbimycins, the TAN-420s, and reblastatin.

Geldanamycin and its derivatives are the most extensively studied of the ansamycins. Although geldanamycin was originally identified as a result of screening for antibiotic activity, current interest in it is based primarily on its cytotoxicity towards tumor cells and, therefore, its potential as an anticancer agent. It is an inhibitor of heat shock protein-90 (“Hsp90”), a chaperone protein that is involved in the folding, activation and assembly of a wide range of proteins (“client proteins”), including key proteins involved in signal transduction, cell cycle control and transcriptional regulation. (Hsp90 exists in a number of isoforms, with the α-isoform being the most common one. For a review on Hsp90 isoforms, see Sreedhar et al., FEBS Letters 562 (1-3), 11-15 (2004). Herein, where reference to a specific isoform is intended, abbreviations such as “Hsp90α” or “Hsp90β” will be used, with “Hsp90” reserved for Hsp90 generically.) The binding of geldanamycin to Hsp90 disrupts Hsp90-client protein interactions, preventing the client proteins from folding correctly and rendering them susceptible to proteasome-mediated destruction. Among the Hsp90 client proteins are many mutated or overexpressed proteins implicated in cancer: p53, Bcr-Abl kinase, Raf-1 kinase, Akt kinase, Npm-Alk kinase p185ErB2 transmembrane kinase, Cdk4, Cdk6, Wee1 (a cell cycle-dependent kinase), HER2/Neu (ErbB2), and hypoxia inducible factor-1α (HIF-1α). However, the hepatotoxicity and poor bioavailability of geldanamycin have lead to its discontinuation as a clinical candidate.
Nevertheless, interest persists in the development of geldanamycin derivatives or analogs (collectively “geldanamycin compounds”) having geldanamycin-like bioactivity, but with a better overall spectrum of properties. Position 17 of geldanamycin has been an attractive focal point, chemically speaking, for the synthesis of geldanamycin compounds because its methoxy group is readily displaced by a nucleophile, providing a convenient entry into 17-substituted-17-demethoxygeldanamycin compounds. Further, structure-activity relationship (SAR) studies have shown that structurally and sterically diverse 17-substituents can be introduced without destroying their ability to bind Hsp90. For exemplary disclosures relating to 17-substituted geldanamycin compounds, see Sasaki et al., U.S. Pat. No. 4,261,989 (1981); Schnur et al., U.S. Pat. No. 5,932,566 (1999); Schnur et al., J. Med. Chem., 38, 3806-3812 (1995); Schnur et al., J. Med. Chem., 38, 3813-3820 (1995); Ho et al., WO 00/03737 A2 (2000); Santi et al., U.S. 2003/0114450 A1 (2003); Zhang et al., WO 03/066005 A2 (2003); and Clevenger et al., J. Org. Chem. 69, 4375-4380 (2004); the disclosures of which are incorporated by reference. The SAR inferences are supported by the X-ray crystal co-structure of the complex between Hsp90α and a geldanamycin derivative (17-DMAG, v. infra), showing that the 17-substituent projects out from the binding pocket and into the solvent (Jez et al., Chemistry & Biology, 10, 361-368 (2003)).
The best-known 17-substituted geldanamycin is 17-allylamino-17-demethoxy-geldanamycin (“17-AAG”), currently undergoing clinical trials. Another noteworthy 17-substituted geldanamycin is 17-(2-dimethylaminoethyl)amino-17-demethoxygeldanamycin (“17-DMAG”), also undergoing clinical trials (Snader et al., WO 02/079167 A1 (2002), incorporated by reference). Like geldanamycin, both 17-AAG and 17-DMAG must be administered with care due to their cytotoxicity.

While most studies concerning the function of Hsp90 have focused on its activity inside cells, there have been a few reports on the extracellular occurrence of Hsp90, usually in association with cancer cells. Eustace et al., Nature Cell Biology, 6 (6), 507-514 (2004, web-published 16 May 2004) (“Eustace et al.”), reported that Hsp90α plays an essential extracellular role in cancer cell invasiveness. They found that fibrosarcoma and breast cancer cells express Hsp90α extracellularly, where it interacts with matrix metalloproteinase-2 (“MMP-2”) and that inhibition of extracellular Hsp90α by geldanamycin decreases both MMP-2 activity and cancer cell invasiveness. Their hypothesis is that matrix metalloproteinases (“MMPs”) are responsible for the degradation of the extracellular matrix, thereby facilitating the invasive action of cancer cells, and that Hsp90α plays a chaperone protein role in the activation of MMPs. Other reported occurrences of extracellular Hsp90 include: Hegmans et al., Am. J. Pathol., 164 (5), 1807-15 (2004); Xu et al., Proc. Natl. Acad. Sci. (USA), 96, 109-114 1999); Xu et al., Proc. Natl. Acad. Sci (USA), 90 7074-7078 (1993); Ferrarini et al., Int. J Cancer, 1992, 613-619; and Pratt, J. Biol. Chem., 268 (29), 21455-21458 (1993).
A drawback to using Hsp90 inhibitors such as geldanamycin, 17-AAG, and 17-DMAG in therapies targeting intracellular Hsp90 is their cytotoxicity, with concommitant lowered therapeutic indices. However, for therapies in which the target is extracellular Hsp90, one can theoretically use Hsp90 inhibitors that do not cross cell membranes and enter cells. If such compounds are still able to bind to and inhibit extracellular Hsp90, their cell impermeability should lead to reduced cytotoxicities and higher therapeutic indices.